1. Field of the Invention
The present invention relates to a superconducting coil system in which energy is supplied from an excitation power source to a superconducting coil through a DC circuit breaker to excite the coil. Particularly, the present invention pertains to a superconducting coil protecting device for protecting the superconducting coil from being damaged due to quenching by dissipating the energy accumulated through a protective resistor connected in a parallel manner to the superconducting coil when the superconducting coil undergoes quenching, as well as a method of operating such a protecting device.
2. Description of the Related Art
Recently, the use of superconducting coils in various apparatuses, such as nuclear fusion devices, accelerators, power energy storage facilities and linear motors, has been greatly increasing. Superconducting coils normally have almost zero electrical resistance. However, they sometimes undergo a transition from a superconductive state to a normal conductive state due to an abrupt change in the magnetic field or a temperature anomaly. This phenomenon involving breakdown of the superconductive state is called quenching. Quenching may cause an increase in pressure due to abnormal evaporation of liquid helium employed for cooling the superconducting coil.
Accordingly, in the conventional superconducting coil system in which energy is supplied from an excitation power source 3 to a load coil, i.e., a superconducting coil, when quenching occurs, the superconducting coil 1 has been protected by dissipating the energy accumulated in the superconducting coil 1 through a protective resistor 2, as shown in FIG. 4, thereby restricting quenching from increasing. That is, once quenching occurs, a looped circuit consisting of the excitation power source 3 and the superconducting coil 1 is interrupted by a DC circuit breaker 4 to shift the current which flows in the looped circuit to the protective resistor 2 connected in a parallel manner to the superconducting coil 1. As a result, the DC circuit breaker 4 which is capable of interrupting a large DC current is required. In the basic superconducting coil system shown in FIG. 4, the portion of the circuit between the protective resistor 2 and the DC circuit breaker 4 is generally called a quenching protecting device.
Recently, the scale of the superconducting coil systems has been expanded. This has resulted in an increase in the current which flows in the superconducting coil 1. Moreover, the number of superconducting coil systems designed for continuous operation has been increasing.
The above developments have made it necessary to have a method of quenching protection which employs a DC circuit breaker 4 capable of interrupting a large DC current and of continuous operation.
However, a single DC circuit breaker is generally designed to handle only a small current, e.g., about 3000 A, and therefore cannot withstand a continuously large current. Accordingly, in the conventional method, a plurality of DC circuit breakers have been connected in a parallel manner, as shown in FIG. 5.
Such conventional protection devices have been disclosed in, for example, Japanese Patent Laid-Open No. 198613/1982.
In this conventional technique, interruption of DC current for quenching protection requires a plurality of DC circuit breakers, making the protection device very expensive.
Furthermore, it is very difficult to simultaneously operate a plurality of parallel-connected DC circuit breakers, and variations in the times of the operations of these circuit breakers may cause damage to the DC circuit breakers or the components which constitute the superconducting coil system.